THE FRESH WATERS 179 



ring of their bodies, and may be seen hanging 

 head downwards from the surface so that air 

 may enter. There are other aquatic larvae 

 which never even get wet. This is difficult to 

 understand, because it requires a knowledge of 

 physical properties, but it is a fact, and it is illus- 

 trated in a very varied way among animals that 

 have gone back from the dry land to the water. 

 Some water-beetles can hardly become wet at 

 all ; some keep the greater part of the body dry, 

 but not it all ; some become quite wet. The 

 little whirligig beetle (Gyrinus) and the Water 

 Boatmen (Notonecta) become very slightly 

 wetted. The water-spider remains dry over 

 a considerable part of the hairy body. 



The time spent in the water is often very 

 long compared with the aerial life. Some of 

 the caddis-flies are said to spend three years 

 in the water, and then only to live a few days. 

 And some aerial lives are shorter still. Some 

 of the May-flies or Ephemeridce, as they are 

 called, from the shortness of their lives, live 

 only a few hours as winged insects in the air. 

 But their larval life in the water lasts for two 

 or three years, during which they feed, grow, 

 and cast their husk many times. At length 

 there is the making of the wings and the 



